FR2706912A1 - - Google Patents

Abstract

Microporous electrically conductive material comprising a fibrous web, free of asbestos, characterized in that said web comprises carbon or graphite fibers, polyfluoroethylene fibers, inert mineral fibers, at least one fluoropolymer binding fibers and at least one thickening. Cathode element comprising said material and association of this cathode element with a diaphragm or a membrane. Use of this combination in a cell for the electrolysis of aqueous solutions of alkali halides.

Description

CATHODIC ELEMENT WITHOUT ASBESTOS FIBERS

  The subject of the present invention is an electrically conductive microporous material that can be used in particular for producing the cathode element of an electrolysis cell, and in particular of an electrolytic cell for alkaline halide solutions. It also relates to the cathode element comprising said material. The invention also applies to the process

manufacturing said material.

  The invention finally relates to the association of said cathode element with a

diaphragm or membrane.

  Finally, it relates to the use of this association in a cell

  electrolysis of aqueous solutions of alkaline halides.

  Aqueous solutions of alkaline halides generally electrolyzed

  are those of sodium chloride to obtain chlorine and soda.

  It is known that the materials used for producing the cathode element of an electrolysis cell must satisfy several qualities: they must have a low electrical resistivity compatible with the operation of the electrolyser equipped with such a cathode element. an acceptable energy level, they must also have a small thickness (from about 0.1 to mm), whereas they must have a large surface that can exceed

several m2.

  In addition, these materials must be obtainable by deposition on a rigid structure which has opening rates and large hole diameters. These materials are generally obtained by vacuum filtration of a

suspension of fibers and binders.

  The properties of such a material depend on a number of parameters, including the nature and concentration of the materials

  fibrous suspension, surfactants, porogens and various additives.

  Thus, to combine these qualities, it is known to use a fibrous material consisting of a mixture of conductive and non-conductive fibers such as in particular described in the European patent application EP 0 319 517. In this patent application, it is more particularly recommended is the mixture of asbestos fibers and carbon fibers, the carbon fibers supplying the cathodic element with electroconductivity and asbestos fibers making it possible to

  bind the binders during filtration.

  Various improvements have been made both to the said material and to their

manufacturing process.

  In the European patent application EP 0 214 066 filed in the name of the Applicant, it has been proposed materials containing carbon fibers having a monodisperse distribution of lengths, materials whose quality and properties are significantly improved, which is translated by

  a much better performance / thickness ratio.

  In the European patent application EP 0 296 076 filed in the name of the Applicant, electroactivated materials containing an electrocatalytic agent uniformly distributed in their mass have been proposed, said agent being chosen from Raney metals and Raney alloys of which one eliminated the

  most of the easily removable metal (s).

  The set of proposed cathode elements which ensure an appreciable distribution of the current can be used in an electrolysis cell which will further comprise a diaphragm or diaphragm separating the anode and cathode compartments. Further technical details can be found in the aforementioned European patent applications. However, the quality of the cathode elements described and known in the state of the art is not fully satisfactory because of the need to use asbestos fibers. Indeed, in addition to the risks associated with the handling of this product that is dangerous for human health, the insufficient chemical stability inherent in asbestos involves various disadvantages, such as a life that is too short for the electrolyser comprising this cathode element and difficulties in modifying the operating conditions of the electrolyser, for example by increasing the electrical intensity and / or the hydroxide concentration

of alkali metal.

  In the patent application EP 0 222 671 filed in the name of the Applicant, there is described in one example the manufacture of a precathode without asbestos from a suspension containing only conductive carbon fibers. However, the composition of the suspension for this production does not make it possible to provide the web with a porosity that is both fine and

  regular and a great cohesion to the cathode.

  It has now been found that it is possible to prepare asbestos-free materials that can be used in particular for producing a cathode element of an electrolysis cell for alkaline halide solutions.

  materials not having the aforementioned drawbacks.

  The present invention therefore relates to an electrically conductive microporous material comprising a fibrous web, free of asbestos, characterized in that said web comprises carbon or graphite fibers, polytetrafluoroethylene fibers, inert mineral fibers, at least one polymer fluorinated binder fibers and at least one thickener. The invention also covers the cathode element comprising said material. Finally, a subject of the invention is a process for the preparation of electroconductive microporous material, characterized in that it comprises the following steps: a) preparation of an aqueous suspension comprising approximately: parts by dry weight of a mixture of fibers consisting of 20 to 80 parts by dry weight of carbon or graphite fibers and with 80 to 20 parts by dry weight of polytetrafluoroethylene fibers; 15. 10 to 100 parts by dry weight of inert mineral fibers; to 60 parts by dry weight of fluoropolymer binding fibers; at 200, preferably from 30 to 100, parts by dry weight of silica-based derivatives; 4.5 to 30 parts by dry weight of at least one thickener; b) depositing a web by vacuum filtration of said suspension through a porous support; c) removing the liquid medium and drying the web thus formed; d) sintering the web;

  e) elimination of silica-based derivatives.

  The electroconductive microporous material capable of being prepared by this process constitutes a preferred electroconductive microporous material of the invention.

present invention.

  According to the invention, the term "electrically conductive microporous material" is intended to mean microporous materials which have an electrical resistivity generally of between 0.5 and 15 Ω.cm. For reasons of energy consumption, this resistivity is preferably between 0.degree. 5 and 10 Q.cm. and

  even more preferably between 0.5 and 2 Q.cm ..

  Even more preferably in the process of the present invention, the mixture of carbon or graphite fibers and polytetrafluoroethylene fibers consists of 60 to 80 parts by dry weight of carbon or graphite fibers and of 40 to 20 parts by weight. in dry weight of polytetrafluoroethylene fibers. Advantageously, the amounts of inert mineral fibers and thickeners used during the process and present in the fibrous web according to the present invention are respectively between 20 and

  parts by dry weight and 5 and 20 parts by dry weight.

  In practice, the aqueous suspension prepared in step a) of the process according to the present invention is a suspension of about 2 to 5% by weight of dry matter. The material, the cathode element and the process for preparing said material according to the invention comprise or implement, as has been stipulated above, a certain number of elements which will now be studied.

  more precisely. For the rest of the description, everything described in

  subject of said elements applies to both the material, the cathode element or the

  diaphragm only to the process that use these elements.

  The polytetrafluoroethylene fibers, hereinafter referred to as PTFE fibers, used in the context of the present invention may have variable dimensions: their diameter (D) is generally between 10 and 500 μm and their length (L) is such that the L / D ratio is between 5 and 500. Preferably, PTFE fibers are used whose average dimensions are between 1 and 10 mm for the length and between 50 and 200 tz m for the diameter. Their preparation is described in US Pat. No. 4

  444 640 and this type of PTFE fibers is known to those skilled in the art.

  The carbon or graphite fibers are in the form of filaments whose diameter is generally less than 1 mm and preferably between 10-5 and 0.1 mm and whose length is greater than 0.5 mm and of

  preferably between 1 and 20 mm.

  Preferably, these carbon or graphite fibers have a monodisperse length distribution, that is to say a length distribution such that the length of at least 80% and, advantageously at least 90%, of the fibers corresponds to average length to + 20%, and

preferably within + 10%.

  By "inert mineral fibers" is meant according to the invention any mineral fiber chemically inert vis-à-vis the products formed during the use of the material in an electrolysis cell. These fibers have the role of providing a consolidation, a strengthening of the diaphragm, but without penalizing the material with respect to its wettability and conductivity. Thus, when using the material in an electrolytic cell for solutions of sodium chloride, the mineral fibers used must be especially

  inert to the soda formed.

  Titanate fibers, calcium sulfoaluminate fibers (ettringite fibers), ceramic fibers (such as zirconium dioxide, silicon carbide, boron nitride fibers), suboxidic fibers are preferably used. titanium of general chemical formula TinO2n-1 with n, integer, between 4 and 10 (of the type Ebonex m manufactured and sold by the company IMI) alone or in admixture. Fibers are more particularly used

of titanate.

  Titanate fibers are known materials. Thus, potassium titanate fibers are commercially available. Other fibers derived from potassium octatitanate K2Ti8O17 by partial replacement of titanium ions at oxidation state IV with magnesium and nickel cations, or at the oxidation state II! such as iron or chromium cations and by charge compensation provided by alkaline ions such as sodium and potassium cations, are described

  in French Patent Application No. 2,555,207.

  Other titanate fibers such as potassium tetratitanate

  (K2Ti409) or its derivatives may be used.

  By "thickener" is meant according to the present invention a compound which increases the viscosity of the solution and which has water retentive properties. Natural or synthetic polysaccharides are generally used. It may be mentioned in particular biopolymers obtained by fermentation of a carbohydrate under the action of microorganisms. The xanthan gum is advantageously used. Xanthan gum is synthesized using bacteria belonging to the genus Xanthomonas and more particularly to the species

  described in Bergey's manual of determining bacteriology (8th edition -

  1974 - Williams N. Wilkins C Baltimore) such as Xanthomonas begoniae, Xanthomonas campestris, Xanthomonas carotae, Xanthomonas hederae, Xanthomonas incanae, Xanthomonas malvacearum, Xanthomonas papavericola, Xanthomonas phaseoli, Xanthomonas pisi, Xanthomonas vasculorum,

  Xanthomonas vesicatoria, Xanthomonas vitians, Xanthomonas pelargonii.

  The species Xanthomonas campestris is particularly suitable for

synthesis of xanthan gum.

  Other microorganisms capable of producing polysaccharides of similar properties include bacteria belonging to the genus Arthrobacter, genus Erwinia, genus Azobacter, genus

  Agrobacter or fungi belonging to the genus Sclerotium.

  Xanthan gum can be obtained by any means known per se.

  Conventionally, the polysaccharide is isolated from the fermentation broth by evaporation, drying and grinding or by precipitation with a lower alcohol, separation of the liquid, drying and grinding so as to obtain a powder. The commercially available powders have a particle size generally of between 50 and 250 μm and a higher bulk density.

at around 0.7.

  The binder of the electrically conductive microporous material according to the invention

  consists of fluorinated polymers.

  By "fluoropolymers" is meant homopolymers or copolymers derived at least in part from olefinic monomers substituted by fluorine atoms, or substituted by a combination of fluorine atoms and at least one

  chlorine, bromine or iodine atoms per monomer.

  Examples of fluorinated homopolymers or copolymers may consist of polymers and copolymers derived from tetrafluoroethylene,

  hexafluoropropylene, chlorotrifluoroethylene, bromotrifluoroethylene.

  Such polymers can also comprise up to 75 mol% of units derived from other ethylenically unsaturated monomers containing at least as many fluorine atoms as carbon atoms, such as for example vinylidene (di) fluoride, esters and the like. vinyl and perfluoroalkyl, such as perfluoroalkoxyethylene. The fluorinated polymer is advantageously according to the invention in the form of an aqueous dispersion containing, in general, from 30 to 70% of dry polymer, with a particle size of between 0.1 and 5 micrometers, and

  preferably between 0.1 and 1 micrometer.

  Preferably, the fluoropolymer used is a polytetrafluoroethylene.

  By "silica-based derivatives" is meant according to the invention the silicas

  precipitated and fumed or pyrogenic silicas.

  Advantageously, the silicas have a BET specific surface area of between 100 m 2 / g and 300 m 2 / g and / or a particle size evaluated by the Coulter counter between 1 and 50 microns and preferably between 1 and 15 microns. These derivatives behave as excellent porogens providing substantially no deconsolidation of the electroconductive microporous material when used in the amounts of the present invention. These derivatives are also agents forming lattices of latex

constituted by the binder present.

  The removal of the silica-based derivatives can be carried out by etching in an alkaline medium. This elimination creates the microporosity of the material according to the present invention. This elimination of the silica-based derivatives can be carried out before the use of the electroconductive microporous material in electrolysis, but, in a practical and advantageous manner, the silica-based drifts can be removed "in situ" in the electrolyser by dissolving alkaline medium especially during the first hours of electrolysis. Thus, the elimination is advantageously carried out in contact with an aqueous solution of sodium hydroxide whose concentration is between 40 and 200 g / l and at a concentration of

  temperature between 20 and 95 C.

  In the present invention, at least one surfactant is preferably added during the preparation of the suspension in step a). The maximum amount of surfactant present is generally 10 parts by dry weight, preferably the amount of surfactant is between 0.5 and 5 parts by dry weight. Preferably, a nonionic surfactant is used. Nonionic surfactants that may be used include ethoxylated alcohols or fluorocarbon compounds with functional groups alone or as a mixture; these alcohols or fluorocarbon compounds generally have carbon chains of C6 to C20. Preferably, ethoxylated alcohols are used which are

  ethoxylated alkylphenols, such as in particular octoxynols.

  Other elements may be added to the suspension of step a) of the process according to the present invention. Thus, these additives may in particular correspond to electrocatalytic agents chosen from the group consisting of Raney metals and alloys from which most of the easily removable metal (s) will be removed, and mixtures thereof. Electroactivated materials containing an electrocatalytic agent of this type are described in

  European Patent Application No. 296,076 filed by the Applicant.

  According to the method of the present invention, the sheet is thus formed by

  programmed vacuum filtration of said suspension through a porous support.

  These porous supports may in particular be webs or grids whose mesh gap, perforations or porosity may be between 1 μm and 5 mm, and preferably between 20 μm and 2 mm. These porous supports may have one or more flat or cylindrical surfaces, called

  commonly "glove finger", having an open surface.

  In the case where the electroconductive microporous material according to the invention is used in the electrolysis cells of alkali halides or more specifically sodium chloride, the electroconductive microporous material may be associated with a metal support which constitutes the elementary cathode. This is more particularly the cathode element or the voluminal electrode. By these terms we mean a rigid metal support whose function is only to bring electricity and a material

  microporous electroconductive which performs the cathodic function.

  This electroconductive microporous material / metal support assembly can be made in different ways. The first way of proceeding is to first manufacture the web as described in steps a) to c) of the process for preparing the electrically conductive microporous material according to the present invention, then to apply this layer on the metal support which constitutes the elementary cathode and then sinter the whole. According to another preferred variant, since it is simpler to use, the suspension described in step a) of the process for preparing the electrically conductive microporous material according to the present invention is filtered directly through the metal support which

constitutes the elementary cathode.

  Thus, the process according to the invention is particularly advantageous in the case where it is desired to prepare a cathode element. This cathode element thus comprises an electroconductive rigid structure, such as a porous metal support, on which is located the electroconductive microporous material.

according to the present invention.

  In the case where the cathode element is used in alkaline halide or more specifically sodium chloride cells, the cathode element may be associated with a diaphragm or a membrane, playing the role of

  separator between the cathode and the anode in an electrolysis cell.

  In the case of a membrane, which may be chosen from the numerous electrolysis membranes described in the literature, the cathode element according to the invention constitutes an excellent mechanical support and ensures a good distribution of the current. This current distribution is linked to the

  particular structure of the cathode element according to the invention.

  The cathode element can therefore also be linked to a diaphragm. The diaphragm consists of microconsolidated fibers in the form of a sheet. This diaphragm, which can also be chosen from the many electrolysis diaphragms now known, can be manufactured separately. It can also, and this is an advantageous modality, be manufactured directly on the tablecloth

  fibers that constitute the electrically conductive microporous material.

  The diaphragms preferably used are those described in the patent applications EP 0 412 916 and 0 412 917 filed by the Applicant. Advantageously used diaphragm without asbestos described

  in the patent application EP 0 412 917.

  It is clear that the association in question is in a way a stack from one side to the other of three layers, namely the porous metal support, the electroconductive microporous material and the membrane or the diaphragm, said

  stack forming a coherent whole.

  The various vacuum programs described previously can be carried out continuously or in stages, from the atmospheric pressure to the final pressure which

  is about 0.01 to 0.5 bar absolute.

  Sintering is generally carried out at a temperature above the melting or softening point of the fluoropolymers, binders of the fibers. This

  sintering allows consolidation of the previously mentioned web.

  In what follows or the above, the percentages expressed are in

weight, unless otherwise stated.

  Concrete but non-limiting examples will now be given.

COMPARATIVE EXAMPLE 1

  Preparation of Microporous Electroconductive Material with Asbestos Fibers A suspension is prepared from: - deionized water, the amount of which is calculated to obtain about 4 liters of suspension and a solids content of about 4.84% by weight; g chrysotyl asbestos fibers whose average length is 1 to 5 mm and whose diameter is about 200 Angstrom,

2.1 g of xanthan gum.

  After rotary stirring for a few minutes, 35 g of polytetrafluoroethylene in the form of a latex with a solids content of 60%, 100 g of precipitated silica in the form of particles having an average particle size of 3 μm and a surface area of B.E.T. is 250 m2.g-1, 70 g of carbon fiber whose diameter is about 10 μm and the average length of which is 1.5 mm, 3.3 g of Triton X 100 from Rohm. and Haas, 121 g of Raney nickel in powder form of 10 tm (marketed Ni)

by the Procatalyse Company).

  After stirring, it is deposited by filtration, under programmed vacuum and preset, on a wire mesh braided and laminated steel type "Gantois" whose opening is 2 mm and whose wire diameter is 1 mm, the surface effective being 1.21 dm2. The depression is established and increases by 50 mbar per minute to about 800 mbar. This maximum depression is maintained during

about 15 minutes.

  The assembly is then dried and then consolidated by melting the fluoropolymer.

  The silica is removed "in situ" in the electrolyser by dissolution in medium

  alkaline especially during the first hours of electrolysis.

  The characteristics of the deposition of the suspension are as follows: the flow time is 200-300 s, the stopping rate is approximately 70%, the final vacuum is 200 to 300 mbar, thickness is 1.5 to 2.5 mm,

  - The electrical resistivity is 0.5 to 2 Q.cm-1.

  The flow time corresponds to the time required to filter the entire suspension (the vacuum being programmed, the flow time becomes a parameter characterizing the deposition and depending on the filtered suspension). The maximum depression reached by the system is maintained for 15

  about minutes, this phase corresponds to the spinning of the sheet thus formed.

  At the end of this spinning phase, the depression is maintained at a stationary level, called final vacuum. This final vacuum is also a parameter characterizing the deposition and depending on the filtered suspension, it is more

  particularly representative of the porous structure of the deposited web.

  The stopping rate is recorded by simple balance of materials by weighing.

  After drying or consolidation, the formed web can be detached from the grid on which the deposit was made. The electrical resistivity can then be measured by means of an ohmmeter connected to two conductive metal plates

  current, themselves being applied to opposite sides of the web.

  Preparation of the combination of the cathode element with asbestos and an asbestos-free diaphragm The cathode element (microporous diaphragm + wire mesh) thus prepared is deposited with a sheet in order to prepare a diaphragm under the conditions given in the examples of patent application EP 0 412 917 filed by

the Applicant.

  The starting suspension for preparing this diaphragm is as follows: 100 g of polytetrafluoroethylene (= PTFE) fibers introduced as g of a mixture of sodium chloride and PTFE fibers (50/50 by weight) treated beforehand, as described below, - 60 g of potassium titanate fibers with a diameter of 0.2 to 0.5 μm and a length of 10 to 20 μm, - 40 g of polytetrafluoroethylene in the form of a latex at approximately 65% by weight of dry extract, - 50 g of silica precipitated in the form of particles of average particle size of 3 μm and whose BET surface is 250 m2 / g,

  3.6 g of triton X 100 from Rohm and Haas.

  The PTFE fibers impregnated with NaCl are pretreated by mixing a solution of one liter of water comprising approximately 100 g of a mixture containing approximately 50% of PTFE fibers and 50% of sodium chloride. This operation is optionally repeated so as to obtain the desired quantity of

PTFE fibers.

  The performance of the combination thus manufactured was then evaluated in an electrolysis cell which has the characteristics and whose operating conditions are indicated below: Expanded titanium anode, rolled, coated with TiO2-RuO2 20. Cathodic element mild steel braided and laminated; 2 mm wire, 2 mm mesh covered with microporous electroconductive material and diaphragm 6 mm anode-combination distance Electrolytic active surface area of 0.5 dm2 Cell assembled according to filter press type 25. Current density 25 A.dm- 2 Temperature of 85 C Constant anodic chloride operation of 4.8 mol.l-1 Title of the electrolytic soda of 120 or 200 g / l The particular conditions and the results obtained are collated in Table (II) below: RF: Faraday output AU: Voltage across the electrolyser below the specified current density At performance (kwh / TCI2) corresponds to the energy consumption of the system in kilowatt hours per ton of chlorine produced 35. AUi -> O corresponds to the electrolysis voltage at 85 C and i = O by extrapolation of the curve U = f (i)

COMPARATIVE EXAMPLE 2

  Preparation of Microporous Electroconductive Material Based on Carbon Fibers An electroconductive microporous material is prepared in the same manner as in Comparative Example 1 from a suspension comprising: 7000 g of permutated water, 100 g of graphite whose length is 1 to 2 mm, 1 g of dioctylsulfosuccinate of Na, 35 g of polytetrafluoroethylene in latex form to about 65% by weight of dry extract, 100 g of precipitated silica in the form of particles of granulometry average of 3 μm and BET surface is 250 m2.g-1

- 235 Raney alloy.

  The characteristics of the deposition of the suspension are the following: the flow time is less than 100 seconds, the stopping rate is about 80%,

  the final vacuum is less than 200 mbar.

  The values of flow time and final vacuum are low. This results in the formation of a thick electroconductive microporous material

  too high and strongly non-uniform throughout its surface.

COMPARATIVE EXAMPLES 3 to 8

  Preparation of Microporous Electroconductive Material Free of Inert Mineral Fibers and Asbestos Fibers An electroconductive microporous material is prepared in the same manner as in Comparative Example 1 from a suspension comprising: - 30 g of PTFE fibers introduced under form of a mixture of sodium chloride and PTFE fibers (50/50 by weight) pre-treated, as previously described, - 70 g of carbon fiber, the diameter of which is approximately 10 μm and the average length of which is 1.5 mm, - 15 or 35 g of polytetrafluoroethylene in the form of latex to about 65% by weight of dry extract, - 50 or 100 g of precipitated silica in the form of particles with an average particle size of 3 μm and whose BET surface is 250 m2.g-1 - Raney nickel

  9 or 16 g of xanthan gum (Rhodopol 23 sold by Rhône-Alpes);

  Poulenc), which corresponds respectively to 0.06 or 0.10% by weight in water

  - O or 3.3 g of triton X 100 from Rohm and Haas.

  The characteristics of the deposit of the suspension are collected in the

following table (I).

  composition of the suspension _ characteristic deposit conditions ex. xanthan latex tritium silica Ni Pds stop rate T (s) VEQ (g) PTFE (g) (g) Raney (g) (%) (mbar) (mm) (Q.cm) _ _ _ (g) (g) _

3 9 15 50 0 5.7 300 62 215 60 2.5 1

  4 9 55 50 0 5.7 302 81 160 83 3 0.5-1

  9 55 100 3.3 5.7 301 40 150 58 2 0.5-1

6 16 35 50 0 300 72 175 70 2 0.5-1

  7 16 35 100 0 0 301 46 175 75 2 0.5-1

  8 16 35 100 3.3 O 301 42 175 85 1 0.5-1

TABLE (I)

  In this table and the following, T represents the flow time, V the final vacuum (see Comparative Example 1), wt. represents the weight deposited on the grid, E

  represents the thickness of the deposit and Q the resistivity.

EXAMPLES 9 to 13

  Preparation of electroconductive microporous material according to the invention In the same manner as in Comparative Example 1, an electroconductive microporous material is prepared from a suspension comprising: - 5, 10, 50 or 100 g of potassium titanate fibers of 0.2 to 1.5 μm in diameter and 10 to 20 μm in length, 30 g of PTFE fibers introduced in the form of a mixture of sodium chloride and treated PTFE fibers (50/50 by weight) beforehand, as previously described, - 70 g of carbon fibers whose diameter is about 10 μm and the average length of which is 1.5 mm, - 15 or 35 g of polytetrafluoroethylene in the form of a latex at about 65% by weight of dry extract, - 50 or 100 g of precipitated silica in the form of particles of average particle size of 3 μm and whose BET surface area is 250 m2.g-1, - Raney nickel 70% dry solids, - 9 g of xanthan gum (Rhodopol 23 marketed by Rhône-Poulenc), which corresponds to 0.06% of mass by weight, respectively. in water,

  2 g of X 100 triton from Rohm and Haas.

  The characteristics of the deposit of the suspension are collected in the

following table (II).

  suspension composition characteristic deposit conditions ex. silica fiber triton Ni Pds stop rate T (s) VEQ titanate PTFE (g) (g) Raney (g) (%) (mbar) (mm) (Q.cm) (g) (g) (g) ) _

  9 5 15 50 2 6.3 300 55 220 100 2.5 7

10 15 50 2 6.3 302 52 215 80 2.5 9

  11 10 15 50 2 6.3 300 60 195 100 2.5 13

  12 50 35 100 2 3 330 51 220 200 1.4 0.5-2

  13 100 35 100 2 2.6 330 37 220 200 1.4 6

TABLE (II)

  These examples show that the addition of inert mineral fibers, such as titanate fibers in microporous materials is necessary to obtain an industrially viable deposit. Indeed, the flow times and final voids obtained are brought to those measured during the manufacture of the material with asbestos fibers described in Comparative Example 1 (unlike Examples 3 to 8 for which the flow time remains insufficient and the

final vacuum too weak).

COMPARATIVE EXAMPLE 14

  Preparation of electroconductive microporous material without xanthan A microporous electroconductive material is prepared in the same manner as in Comparative Example 1 from a suspension comprising: - 50 g of potassium titanate fibers of diameter 0.2 to 1.5 pm and length of 10 to 20 μm, 30 g of PTFE fibers introduced in the form of a mixture of sodium chloride and PTFE fibers (50/50 by weight) treated beforehand, as described above, 70 g of carbon fibers, the diameter of which is approximately 10 μm and the average length of which is 1.5 mm, 35 g of polytetrafluoroethylene in the form of latex at approximately 65% by weight of solids, 100 g of silica precipitated in the form of particles of average particle size of 3 μm and whose BET surface is 250 m2.g-1, - 3 g of Raney nickel 70% solids, The characteristics of the deposition of the suspension are as follows: the weight of the filtered suspension is 330 g, the time of the flow rate is 120 s, the stopping rate is approximately 50%, the final vacuum is 163 mbar, the thickness is 1.3 to 1.5 mm,

  the electrical resistivity is 10 Ω.cm.

  EXAMPLES 15 and 16 Preparation of Microporous Electroconductive Material According to the Invention The conditions of Example 12 described above are repeated, only the amount of Raney nickel is modified. In the suspension, 5 (Example 15) or 2.5 (Example 16) were charged with Raney nickel. With the grid, the electrically conductive microporous material constitutes a cathodic element without

asbestos according to the invention.

  Preparation of the Combination of the Non-Asbestos-Free Cathode Element and an Asbestos-free Diaphragm The conditions of Comparative Example 1 were used to obtain this combination. The performances of these associations are gathered in the table

next (ll1).

  Ex. Diaphragm material Performance in microporous electrolysis pds wt. RF NaOH RF> AUC kwh / TCI2 H2 (%) (Kg / m2) (Kg / m2) (g / l) (%) (Volt) (Volt) in chlorine

  0.28 1.95 120 98.5 3.05 2.19-2.20 2340 <0.2

96 3.05 2.19-2.20 2400 <0.2

92 3.05 2.19-2.20 2500 <0.2

88 3.05 2.19-2.20 2620 <0.2

  ______ l ___ _ 200 86 3.05 2.19-2.202680 <0.2

  16 0.34 1.97 120 98 3.2 2.20-2.22 2465 <0.2

95 3.2 2.20-2.22 2540 <0.2

92 3.2 2.20-2.22 2625 <0.2

88 3.2 2.20-2.22 2745 <0.2

  ____________200 86 3.2 2.20-2.22 2810 <0.2

  1 0.35 2 120 98.5 3.05 2.19-2.20 2340 <0.2

96 3.05 2.19-2.20 2400 <0.2

92 3.05 2.19-2.20 2500 <0.2

88 3.05 2.19-2.20 2620 <0.2

  _______ _____ 200 86 3.05 2.19-2.20 2680 <0.2

  It is noted that the performance of the associations according to the invention are satisfactory while having low levels of hydrogen in chlorine. These performances are comparable to an element association

  cathode and diaphragm with asbestos (Comparative Example 1).

Claims (24)

  An electroconductive microporous material comprising a fibrous web, free of asbestos, characterized in that said web comprises carbon or graphite fibers, polyfluoroethylene fibers, mineral fibers   inert, at least one fluoropolymer binding fibers and at least one thickener.   2. Process for the preparation of electroconductive microporous material, characterized in that it comprises the following steps: a) preparation of an aqueous suspension comprising approximately: parts by dry weight of a fiber mixture consisting of 20 to 80 parts by weight dry carbon fiber or graphite and 80 to 20 parts by dry weight of polytetrafluoroethylene fibers; to 100 parts by dry weight of inert mineral fibers; to 60 parts by dry weight of fluoropolymer binding fibers; at 200, preferably from 30 to 100, parts by dry weight of silica-based derivatives; 20. 4.5 to 30 parts by dry weight of at least one thickener; b) depositing a web by vacuum filtration of said suspension through a porous support; c) removing the liquid medium and drying the web thus formed; d) sintering the web;   e) elimination of silica-based derivatives.   3. Method according to claim 2, characterized in that the mixture of carbon fibers or graphite and polytetrafluoroethylene fibers consists of 60 to 80 parts by dry weight of carbon or graphite fibers and   by 40 to 20 parts by dry weight of polytetrafluoroethylene fibers.   4. Method according to one of claims 2 or 3, characterized in that the   the amount of inert mineral fiber is from 20 to 60 parts by dry weight.   5. Method according to one of claims 2 to 4, characterized in that the   amount of thickener is between 5 and 20 parts by dry weight.   6. Method according to one of claims 2 to 5, characterized in that the   aqueous suspension prepared in step a) is a suspension at about 2 to 5% by weight of dry matter.   7. Method according to one of claims 2 to 6, characterized in that the   polytetrafluoroethylene fibers have a diameter (D) of between 10 and   500 μm and a length (L) such that the L / D ratio is between 5 and 500.   8. Method according to the preceding claim, characterized in that the polytetrafluoroethylene fibers have average dimensions between   1 and 10 mm for the length and between 50 and 200 [im for the diameter.   9. Method according to one of claims 2 to 8, characterized in that the   carbon fibers or graphite are in the form of filaments whose diameter is less than 1 mm and preferably between 10-5 and 0.1 mm and whose length is greater than 0.5 mm and preferably , between 1 and mm.   10. Method according to one of claims 2 to 9, characterized in that the   Carbon fibers or graphite have a monodisperse length distribution.   11il. Process according to one of Claims 2 to 10, characterized in that the   inert mineral fibers are selected from titanate fibers, calcium sulfoaluminate fibers, ceramic fibers, titanium suboxide fibers of general chemical formula TinO2n-1 with n, whole number, inclusive   between 4 and 10, alone or mixed.   12. Method according to the preceding claim, characterized in that the fibers   Inert minerals are titanate fibers.   13. Method according to one of claims 2 to 12, characterized in that   the thickener is a natural or synthetic polysaccharide.   14. Method according to the preceding claim, characterized in that   the thickener is a xanthan gum.   15. Process according to any one of Claims 2 to 14, characterized in   that the fluorinated polymers are polymers and copolymers derived from tetrafluoroethylene, hexafluoropropylene, chlorotrifluoroethylene, bromotrifluoroethylene, these polymers may also comprise up to 75 mol% of units derived from other ethylenically unsaturated monomers containing at least as many fluorine atoms as of carbon atoms, such as vinylidene (di) fluoride, vinyl esters and   perfluoroalkyl, such as perfluoroalkoxyethylene.   16. Method according to the preceding claim, characterized in that the   fluoropolymer is a polytetrafluoroethylene.   Method according to one of claims 2 to 16, characterized   what is added, when preparing the suspension in step a), at least a surfactant.   18. Process according to the preceding claim, characterized in that the maximum amount of surfactant present is 10 parts by dry weight, preferably the amount of surfactant is between 0.5 and 5 parts by weight. dry.   19. Process according to any one of Claims 2 to 18, characterized in   the porous support is a metal support.   20. An electroconductive microporous material according to claim 1, characterized in that it is obtainable by the method according to one of the following:   any of claims 2 to 19.   21. cathode element, characterized in that it comprises a rigid electroconductive structure on which the microporous material is located   electrically conductive according to claim 20.   22. cathode element according to the preceding claim, characterized in that the electroconductive rigid structure is the porous support used in the preparation of the electrically conductive microporous material according to claim 20.   23. Association of the cathode element according to one of claims 21 or   22 with a diaphragm or membrane.   24. Association according to the preceding claim, characterized in that the diaphragm is a diaphragm without asbestos.   25. Use of the combination according to one of claims 23 or 24 in a   electrolysis cell of alkaline halide solutions.